1. Field of the Invention
The present invention generally relates to a calculus disintegrating apparatus and a method thereof capable of focusing shock waves onto a calculus or concretion located within a biological body under medical examination so as to destroy or disintegrate the calculus. More specifically, the present invention is directed to a calculus disintegrating method and apparatus with an automatic threshold value setting function for shock waves.
2. Description of the Prior Art
Various calculus disintegrating or destroying apparatuses have been widely utilized in the medical field, in which shock waves generated outside a biological body under medical examination are focused onto calculi or concretions present in an organ (e.g., kidney) in order to disintegrate or destroy these calculi. It is well known, as a shock wave generating source, employments of an electrode discharge, an electromagnetic induction plate or an explosive compound (blasting powder). Very recently, a piezoelectric element has been employed as the shock wave generating source, since such a calculus disintegrating apparatus with employment of the piezoelectric elements owns a particular advantage, for instance, compactness, low cost and less consuming products.
In general, shock wave energy caused by shock waves focused onto a focal point has a sufficiently high value capable of disintegrating or destroying calculi or concretions. In addition, however, if the shock waves are mistakenly focused onto a soft tissue around the calculi, there are risks that hematoma and the like are formed in this soft tissue, i.e., harmful side effects. Under such circumstances, controlling methods to precisely project the shock waves only to the calculi, have been demanded.
Generally speaking, such controlling methods have been proposed: Prior to a shock-wave application, ultrasonic pulses having low energy (signal levels) are previously irradiated to the biological body and reflection waves (echoes) from an area near a focal point are detected. Thereafter, a judgement is made whether or not a focusing operation can be achieved by comparing the signal strengths of the echo signals with a predetermined value. That is, when the signal strength of the echo signals is greater than this value, it can be judged that the ultrasonic waves are focused onto the calculi, whereby shock waves can now be irradiated onto the calculi.
As is known, since an acoustic impedance of a calculus is higher than that of a soft tissue positioned near this calculus, a strength (level) of reflection (echo) signal from the calculus is higher than that from the soft tissue. Accordingly, it can be recognized that when a reflection signal having a high signal strength is received, the ultrasonic pulses are focused onto a calculus. Conversely, when a reflection signal having a low signal strength is received, since the ultrasonic pulses are not focused onto the calculus (namely, focused onto the soft tissue near the calculus), irradiation of shock waves is interrupted.
FIG. 1 shows one conventional calculus disintegrating apparatus utilizing the above-described characteristics of reflection signals, which is described in, for instance, U.S. Pat. No. 4,819,621 to Ueberle et al, entitled "METHOD FOR DETECTION OF CAVITATIONS DURING MEDICAL APPLICATION OF HIGH SONIC ENERGY".
In the conventional calculus disintegrating apparatus of FIG. 1, a piezoelectric element 1 having a shape of a spherical cup is employed. The piezoelectric element 1 functions as a shock wave generating source. At a center hole 1A of this piezoelectric element 1, an ultrasonic imaging probe 2 is mounted by which the shock waves may be focused onto a calculus 3. That is, the ultrasonic probe 2 performs an ultrasonic scanning operation with respect to a region covering this calculus 3 to acquire echo signals. The echo signals are processed so as to reconstruct an ultrasonic image of this scanned region in an ultrasonic diagnostic apparatus 10 and the reconstructed image is displayed on a TV monitor 11. As a result, an operator can control the shock waves to be focused onto the calculus 3, while observing this ultrasonic image displayed on the TV monitor 11.
There are two different types of power sources, i.e., a high-voltage power source 7 and a low-voltage power source 8. A pulser 4 is connected via a voltage selection switch 12 to these power sources 7 and 8. When the high-voltage power source 7 is connected via the switch 12 to the pulser 4, shock waves are generated from the piezoelectric element 1. Also, when the low-voltage power source 8 is connected via the switch 12 to the pulser 4, ultrasonic pulses with low levels which never produce such shock waves are generated from this piezoelectric element 1. Either the shock waves or the ultrasonic pulses generated from the piezoelectric element 1 are reflected from the calculus 3 and are returned as echoes to the piezoelectric element 1, so that these echoes are converted into electric signals which are then supplied to a receiver 5.
When the ultrasonic pulses are generated by energizing the piezoelectric element 1 under the low-voltage power source 8, the receiver 5 detects the echoes thereof to obtain the reflection signals. The reflection signals are then supplied to a judging circuit together with a preset threshold value derived from a threshold value setting circuit 13. The judging circuit 6 compares the above reflection signals with this threshold value. When the level of the reflection signal is higher than the threshold value, the judging circuit 6 makes a decision that the ultrasonic pulses generated from the piezoelectric element 1 are just focused onto the calculus 3. Accordingly, the judging circuit 6 controls the switch 12 in order that the pulser 4 is selectively connected from the low-voltage power source 8 to the high-voltage power source 7. As a result, the shock wave irradiation from the piezoelectric element 1 to the calculus 3 is now prepared. A display output circuit 9 is provided, whereby an image produced from the reflection signals obtained by the receiver 5 is displayed on the TV monitor 11.
In the above-described conventional calculus disintegrating apparatus, there are the following drawbacks. Since there are variations in sizes, shapes and positions of calculi, depending upon individual patients, even when the shock waves having the same or similar strengths are irradiated to the calculi or concretions, strengths (levels) of reflection signals obtained by the receiver 5 are different from each other. Under such circumstances, the threshold value is not always determined as a constant value. Therefore, the threshold value setting operations by way of the threshold value setting circuit 13 must be carried out every time the patients are diagnosed, which then gives heavy workloads to the operator.
To the contrary, if such a threshold value setting operation would not be performed for several patients, erroneous judgments could be made. That is, for instance, although the ultrasonic pulses are surely focused onto the calculus 3, the judging circuit 6 never instructs that the low-voltage power source 8 should be turned OFF and the high-voltage power source 7 should be turned ON so that no shock wave is generated from the piezoelectric element 1. In the worst case, since the shock waves would be irradiated to a normal tissue which need not be cured, this normal tissue could be destroyed, which would cause a serious medical problem.